1.  Desert
   If you're interested in desert ecosystems and you haven't been
there yet, you absolutely HAVE to visit the Arizona-Sonora Desert
Museum (http://www/desertmuseum.org), with its rich displays of desert
flora and fauna.

   If you prefer a more unstructured hike in the desert, Sabino Canyon
Recreation Area is a part of the Coronado National Forest and provides
a variety of hiking trail ranging from paved and pleasant, to
primitive and challenging.

2.  Air and Space
   The Pima Air & Space Museum (http://pimaair.org) is the largest Air
& Space Museum in the Western United States, with a vast collection of
military and civilian aircraft.

   The Titan Missile Museum in nearby Green Valley is the only
publicly accessible Titan II missile site in the nation. When you
visit the Titan Missile Museum, you travel through time to stand on
the front line of the Cold War.

3.  Science
   Kitt Peak National Optical Astronomy Observatory.  Located 56 miles
southwest of Tucson on the Tohono O'odham Indian Reservation, the
observatory is a working research station situated high above on Kitt
Peak.

4.  History
   Mission San Xavier del Bac.  Built in the 1700s, this recently
restored white adobe church, representing a wonderful example of
Spanish colonial architecture, is beautiful inside and out, with
colorful frescoes, sculptures and carvings.